Photographic elements which produce images having an optical density directly related to the radiation received on exposure are said to be negative working. A positive photographic image can be formed by producing a negative photographic image which is a negative of the first negative--that is, a positive image. A direct positive image is understood in photography to be a positive image that is formed without first forming a negative image. Direct positive photography is advantageous in providing a more straightforward approach to obtaining positive photographic images.
While silver halide photography offers the highest attainable photographic speeds for direct positive imaging, there are within the field of silver halide photography a surprising number of different approaches for direct positive imaging. A number of these approaches are reviewed in James, The Theory of the Photographic Process, 4th Ed., Macmillan 1977, Chapter 7, "Latent Image Effects Leading to Reversal and Desensitization".
Internal latent image desensitization type imaging is known to produce the highest attainable photographic speeds among the various direct positive silver halide imaging approaches. According to this approach a silver halide emulsion is employed containing internal latent image forming silver halide grains which are substantially free of surface fog. After imagewise exposure, the silver halide grains are developed with a surface developer--that is, one which will leave the latent image sites within the silver halide grains substantially unrevealed. Simultaneously, either by uniform light exposure or by the use of a nucleating agent, the silver halide grains are subjected to development conditions that would cause fogging of surface latent image forming silver halide grains. The internal latent image forming silver halide grains which received actinic radiation during imagewise exposure develop under these conditions at a slow rate as compared to the internal latent image forming silver halide grains remaining.
Stauffer U.S. Pat. No. 2,497,917 recognized that certain antifoggants when employed in internal latent image desensitization direct positive imaging not only reduce fog in minimum density areas, but also increase maximum density. Subsequently it has become the accepted practice in the art to employ maximum density enhancing antifoggants in internal latent image desensitization direct positive imaging. This special class of antifoggants are known to be useful whether incorporated directly in the photographic element or incorporated in a processing solution, such as a developer. Applications of maximum density enhancing antifoggants to more modern forms of internal latent image desensitization direct positive imaging are illustrated by Evans U.S. Pat. No. 3,761,276 and Research Disclosure, Vol. 151, November 1976, Item 15162. Research Disclosure is published at Emsworth Studios, 535 West End Avenue, New York, N.Y. 10024. Internal latent image desensitization direct positive imaging to produce silver images is specifically illustrated by Hoyen and Silverman U.S. Pat. Nos. 4,444,874 and 4,444,865.
Direct positive silver halide emulsions exhibit art recognized disadvantages as compared to negative working silver halide emulsions. Although internal latent image desensitization imaging is the highest speed approach to direct positive imaging with silver halide emulsions, direct positive photographic speeds are still not high as compared to those achieved routinely with negative working silver halide emulsions. Thus, there is a need in the art for improvements in the photographic speed of this imaging approach.
A second disadvantage of internal latent image desensitization direct positive imaging is that rereversal occurs on overexposure.
A schematic illustration of rereversal is provided in FIG. 1, which plots density versus exposure. A characteristic curve (stylized to exaggerate curve features for simplicity of discussion) is shown for a direct positive emulsion. When the emulsion is coated as a layer on a support, exposed, and processed, a density is produced. The characteristic curve is the result of plotting various levels of exposure versus the corresponding density produced on processing. At exposures below level A underexposure occurs and a maximum density is obtained which does not vary as a function of exposure. At exposure levels between A and B useful direct positive imaging can be achieved, since density varies inversely with exposure. If exposure occurs between the levels indicated by B and C, overexposure results. That is, density ceases to vary as a function of exposure in this range of exposures. If a subject to be photographed varies locally over a broad range of reflected light intensities, a photographic element containing the direct positive emulsion can be simultaneously exposed in different areas at levels less than A and greater than B. The result may, however, still be aesthetically pleasing, although highlight and shadow detail of the subject are both lost. If it is attempted to increase exposure for this subject, however, to pick up shadow detail, the result can be to increase highlight exposure to levels above C. When this occurs, rereversal is encountered. That is, the areas overexposed beyond exposure level C appear as highly objectionable negative images, since density is now increasing directly with exposure. Useful exposure latitude can be increased separating exposure levels A and B, but this is objectionable to the extent that it reduces contrast below optimum levels for most subjects. Therefore reduction in rereversal is most profitably directed to increasing the separation between exposure levels B and C so that overexposed areas are less likely to produce negative images. (In actual practice the various segments of the characteristic curve tend to merge more smoothly than illustrated.)
In silver halide photographic elements which produce dye images for viewing the form of the developed silver is of little concern, since the silver produced on development is an unwanted by-product of dye formation and is generally bleached from the photographic element. However, in black and white photography the usual practice is to rely on developed silver wholly or partially to produce the viewable image. The form of the silver produced on development can have important effects on image quality and the amount of silver required. One measure of the efficiency of silver use in black and white imaging is covering power. Although expressed in various units in the art, as herein employed covering power is defined as 100 times the ratio of maximum density to silver, expressed in grams per square decimeter. From this definition it is apparent that achieving an increase in the maximum density of a silver image without increasing silver coverage is expressed more succinctly as increasing covering power.